Mechanistic Study of Enhanced Protonation by Chromium(III) in Electrospray Ionization: A Superacid Bound to a Peptide

Abstract

Addition of trivalent chromium, Cr­(III), to solutions undergoing electrospray ionization (ESI) enhances protonation and leads to formation of [M + 2H]2+ for peptides that normally produce [M + H]+. This effect is explored using electronic structure calculations at the density functional theory (DFT) level to predict the energetics of various species that are potentially important to the mechanism. Gas- and solution-phase reaction free energies for glycine and its anion reacting with [Cr­(III)­(H2O)6]3+ and for dehydration of these species have been predicted, where glycine is used as a simple model for a peptide. For comparison, calculations were also performed with Fe­(III), Al­(III), Sc­(III), Y­(III), and La­(III). Removal of water from these complexes, as would occur during the ESI desolvation process, results in species that are highly acidic. The calculated pK a of Cr­(III) with a single solvation shell is −10.8, making [Cr­(III)­(H2O)6]3+ a superacid that is more acidic than sulfuric acid (pK a = −8.8). Binding to glycine requires removal of two aqua ligands, which gives [Cr­(III)­(H2O)4]3+ that has an extremely acidic pK a of −28.8. Removal of additional water further enhances acidity, reaching a pK a of −84.7 for [Cr­(III)­(H2O)]3+. A mechanism for enhanced protonation is proposed that incorporates computational and experiment results, as well as information on the known chemistry of Cr­(III), which is substitutionally inert. The initial step involves binding of [Cr­(III)­(H2O)4]3+ to the deprotonated C-terminus of a peptide. As the drying process during ESI strips water from the complex, the resulting superacid transfers protons to the bound peptide, eventually leading to formation of [M + 2H]2+.

Solvent-Mediated Proton-Transfer Catalysis of the Gas-Phase Isomerization of Ciprofloxacin Protomers

“…Electrospray ionization (ESI) is one of the most common mass spectrometry ionization methods for liberating analytes into the gas phase and, if required, furnish them with charge. In the case of protonation, the ESI mechanisms are still the subject of ongoing research. − Since the analyte molecules transition from solution phase to gas phase, with a charged-droplet intermediate state, various protonation sites can be plausibly implicated, even for simple small molecules. ,, A commonly accepted ESI mechanism for low molecular weight species is the ion evaporation model, which posits that protonated analyte molecules are ejected from charged nanodroplets of solvent into the gas phase . Other mechanisms for protonation include the charged residue model, which is typically associated with larger globular species, where the solvent shell evaporates to completion, leaving the remaining charges to be carried in the gas phase by the analyte. , Finally, the chain ejection model has recently emerged as a mechanism that applies to larger denatured proteins and polymer chains, where sequential addition of charge to the analyte at the surface of the droplet drives the ejection of the multiply charged ions. − …”